In order to increase the lamination factor of a coil, alignment winding is conventionally performed in which a wire is wound in multiple layers with adjacent wires set in tight contact with each other. In the alignment winding, however, there is a problem that a winding becomes loose due to variation in wire diameter or bobbin dimension, which lowers the lamination factor of a coil thus failing to achieve an adequate magnetomotive force.
There are a number of methods of performing alignment winding. For example, one flange of a bobbin is arranged to be slidable thereby allowing the axial dimension of a spool portion of the bobbin to flexibly vary so that a plurality of coil sections each set in multiple layers can be axially aligned (refer, for example, to Japanese Utility Model Application Publication No. H7-041132).
FIG. 1 shows a bobbin 30 disclosed in the aforementioned Japanese Utility Model Application Publication No. H7-041132.
Referring to FIG. 1, the bobbin 30 includes a spool portion 31, a stationary flange 32 fixedly disposed at one end of the spool portion 31, a movable flange 33 disposed axially slidable over the spool portion 31, and a stopper 34 formed at the other end of the spool portion 31 and adapted to prevent the movable flange 33 from dropping out.
Referring to FIG. 2 showing a winding process of a wire 35 on the bobbin 30, the movable flange 33 is set at a portion of the spool portion 31 so as to provide a distance equivalent to an integral multiple number of the diameter of the wire 35 from the stationary flange 32, a lead-out line of the wire 35 is soldered to a terminal pin 38 implanted in the stationary flange 32, and the wire 35 is alignment-wound in multiple layers around the spool portion 31 at the distance provided between the stationary flange 32 and the movable flange 33 by means of an arm 36 of an NC-controlled winding machine (not shown) thus a coil segment 37 is formed. Then, the movable flange 33 is slid toward the stopper 34 to provide the aforementioned distance from the end of the coil segment 37, and the wire 35 is alignment-wound in the same way thereby forming another coil segment 37. By repeating the process described above, a plural number of the coil segments 37 are axially arranged in a contact manner. If the wire 35 is a fusing wire, a molten resin coated on the surface of the wire 35 is fused by a heat after the winding process and cooled for solidification.
With the provision of the movable flange 33 as disclosed in the Japanese Utility Model Application Publication No. H7-041132 by which the variation of a wire diameter is absorbed at the process of forming a coil, the bobbin 30 described above allows the plural coil segments 37 to be axially arranged solidly without providing partitions thus increasing the coil lamination factor.
Another method for alignment winding is conventionally performed by using a wire winding tool including a spindle and a pair of circular cylindrical wire holders disposed to be freely telescoped over the spindle, such that the distance between the opposing faces of the pair of wire holders are appropriately set whereby alignment winding is achieved in multiple layers with a high accuracy (refer, for example, to Japanese Patent Application Laid-Open No. H4-042757).
FIG. 3 shows a bobbin 45 set on a wire winding tool 40 disclosed in the aforementioned Japanese Patent Application Laid-Open No. H4-042757, and FIG. 4 is an axial cross sectional view of the same.
The wire winding tool 40 includes a spindle 41 and a pair of wire holders 42a and 42b shaped circular cylindrical and disposed to be freely telescoped over the spindle 41. The diameter of the spindle 41 is substantially equal to or a slightly smaller than the inner diameter of a spool portion 46 of the bobbin 45. One wire holder 42a is disposed stationary, and the other wire holder 42b is disposed to be freely movable in the axial direction.
The bobbin 45 integrally includes the aforementioned spool portion 46 and a protrusion 47 disposed at one end of the spool portion 46 so as to protrude radially outwardly and adapted to function as a rotation stopper and as a terminal pin block.
The bobbin 45 is put on the spindle and telescoped thereover so that the protrusion 47 fits flush into a recess 43 of the wire holder 42a. Then, the wire holder 42b is telescoped over the spindle 41 so as to provide a predetermined distance (m) from the wire holder 42a, one end of a self-fusing wire W is wrapped around one of two terminal pins 48 implanted in the protrusion 47, and the wire W is wound around the spool portion 46 thereby performing alignment winding.
In the alignment winding method disclosed in the Japanese Utility Model Application Publication No. H7-041132, while the variation of the diameter of the wire or the dimension of the spool portion 31 can be absorbed, the space for winding the wire 35 is lessened by the presence of the stationary flange 32 and the movable flange 33, and this is crucial when the bobbin 30 is downsized for use in a small motor.
Recently, a stepping motor is used more and more extensively because it can be controlled easily, and with the downsizing and the enhanced performance of a device, the stepping motor for use in such the device is also required to be downsized. For example, a stepping motor with a diameter of 6 mm is used in a compact digital camera. Accordingly, the winding space of the small stepping motor is inevitably limited thus failing to generate an adequate magnetomotive force, which results in failure to achieve a sufficient torque.
On the other hand, the alignment winding method disclosed in the Japanese Patent Application Laid-Open No. H4-042757 requires the wire winding tool 40 including the wire holders 42a and 42b of high precision.
While the variation of the diameter of the wire W and the variation of the dimension of the spool portion 46 of the bobbin 45 can be absorbed by adjusting the distance (m) defined between the wire holders 42a and 42b of the wire winging tool 40, the wire holders 42a and 42b have their bore diameter set substantially equal to or slightly larger than the outer diameter of the spool portion 46 so that they can be engagingly telescoped over a portion of the spool portion 46, whereby end portions of the spool portion 46 are occupied by the wire holders 42a and 42b during the process of winding, and therefore the space for winding the wire W is axially restricted. Consequently, the magnetomotive force generated by the resulting coil formed on the spool portion 46 of the bobbin 45 is also restricted.
Also, the resulting coil has its axial dimension smaller than the length of the spool portion 46 leaving an open space at the end portions of the spool portion 46 and may possibly be allowed to undesirably move in the axial direction, for example, at the time of assembly process. Further, the coil does not have flanges or like members thus allowing its both end faces to be substantially exposed, and therefore may possibly be loosened, deformed or damaged at the time of assembly process and the like.